SLAC-PUB-10430 LCLS-TN-04-6, April, 2004 Temporal pro¯le of the LCLS photocathode ultraviolet drive laser tolerated by the microbunching instability¤ Juhao Wu,y P. Emma, Z. Huang, and C. Limborg Stanford Linear Accelerator Center, Stanford University, Stanford, CA 94309 M. Borland Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439 The high quality LCLS electron beam generated in the photoinjector is subject to many insta- bilities in the downstream acceleration and compression. The instability can be initiated by any density modulation of the electron beam generated at the photocathode. In this paper, we prescribe the tolerance on the initial electron beam density modulation possibly introduced by the ultraviolet (uv) laser at the cathode. Our study shows that with a matched Landau damping laser-heater, the initial rms density modulation of the electron beam at the photocathode must be less than 5% to ensure the FEL lasing and saturation. Submitted to the 22nd International Linear Accelerator Conference (LINAC 2004) PACS numbers: 29.27.Bd, 41.60.Cr; 52.38.-r; 52.59.Sa Keywords: Beam dynamics; Collective e®ects and instabilities; Free-electron lasers; Laser-plasma interac- tions; Space-charge-dominated beams I. INTRODUCTION Landau damping such that downstream instabilities can be suppressed. Density and energy modulations could be initiated by The success of the free-electron laser (FEL) calls for a shot noise in the electron beam born at the photocath- high quality electron beam characterized by high peak ode. Such shot noise is normally small. But temporal current, small slice emittance, and small slice energy modulation on the ultraviolet (uv) laser pulse itself can spread. However, such a high quality electron beam gen- be transferred to the electron beam at birth. As dis- erated from a photocathode gun is subject to various cussed above, any initial electron beam density or energy impedance sources along the downstream acceleration modulations can be ampli¯ed. If this happens, the FEL and compression. This can lead to beam quality degra- lasing may be a®ected. Hence, we need to specify the tol- dation and beam instability. Speci¯cally, the impedance erance of any density modulations of the electron beam (associated with space charge, wake¯eld, and coherent generated from the photocathode of the gun. synchrotron radiation) and momentum compaction fac- The approach we take is to ¯rst set the requirements tor in the linac and compressors act as an ampli¯er for on the electron beam quality which we need at the en- initial density and energy modulations. Since the slice trance of the undulator. We then ask what tolerance on emittance and slice energy spread are extremely small, the density modulation of the electron beam is needed at Landau damping is not e®ective in suppressing all these the initial photocathode to make sure that the require- potential instabilities. Such instabilities can increase ment we look for at the entrance of the undulator can be the slice energy spread and emittance, and therefore de- satis¯ed. grade FEL lasing. FEL operation calls for best achiev- able beam quality; yet, unnecessarily high quality ren- ders it more susceptible to instabilities associated with II. SYSTEM PARAMETERS WITH beamline impedance and momentum compression. To LASER-HEATER address this quandary, a laser-heater [1] is introduced into the Linac Coherent Light Source (LCLS) beamline. In our study, we take the nominal LCLS accelerator The laser-heater is composed of a short undulator with system setup with the laser-heater included. The LCLS resonant laser-electron energy modulation in a weak mag- FEL parameters are listed in Table I. A schematic of the netic chicane (see [1]). It is designed to be an adjustable LCLS layout is shown in Fig. 1 indicating the location of control, which will impose a limited increase on the slice laser-heater. The laser-heater is to be installed upstream energy spread to the level where FEL lasing is still guar- DL1 (the ¯rst dog-leg transport line in Fig. 1) where the anteed. This `procured' increase is designed to enhance electron beam energy is at E = 135 MeV. Parameters for the laser-heater are listed in Table II. In our study, we used a total temporal compression factor of 30. Hence, ¤Work supported by the U.S. Department of Energy under contract if we require the rms slice relative energy spread to be ¡4 DE-AC03-76SF00515 about 1 £ 10 when the beam energy reaches E = 14:1 yEmail:[email protected] GeV at the entrance of the undulator, then the laser- 2 TABLE I: Main parameters for the LCLS FEL. TABLE II: Main parameters for the LCLS laser-heater. Parameter Symbol Value Parameter Symbol Value electron energy γmc2 14.1 GeV electron energy γ mc2 135 MeV bunch charge Q 1 nC 0 average beta function ¯x;y 10 m bunch current If 3.4 kA n transverse rms beam size σx;y 190 ¹m transverse norm. emittance "x;y 1 ¹m undulator period ¸u 0.05 m average beta function ¯x;y 25 m undulator ¯eld B 0.33 T undulator period ¸u 0.03 m undulator parameter K 1.56 undulator ¯eld B 1.3 T undulator length L 0.5 m undulator parameter K 3.64 u laser wavelength ¸L 800 nm undulator length Lu 130 m laser rms spot size σr 175 ¹m FEL wavelength ¸r 1.5 Aº laser peak power P 1.2 MW FEL parameter ½ 4:8 £ 10¡4 L Rayleigh range ZR 0.6 m 2 maximum energy modulation ¢γL(0)mc 80 keV 2 rms local energy spread σγL mc 40 keV heater should give a maximum rms slice energy spread of σE = 47 keV, assuming that the longitudinal phase space area is conserved. We note that there are multiple sources of dilution to the longitudinal phase space area. for the microbunching wavelength ¸ 2 [50; 300] ¹m, the This amount of slice energy spread then determines the space charge oscillation period is about 1:5 to 3:5 m, if Landau damping strength to suppress instabilities in the the electron beam energy is around 10 MeV. So, for typ- accelerator system. Therefore, this with laser-heater in ical device of length of a few meters in the beam line, place imposes a limit on the maximum amount of den- there would be space charge oscillation [1]. The space- sity modulations in the electron beam born at the laser charge oscillation period is a sensitive function of beam cathode of the injector. energy. For the same parameter set, the oscillation pe- The laser-heater introduces energy modulation at the riod is 10 to 45 m for a 30 MeV energy beam. Therefore laser wavelength of 800 nm. A schematic of the laser- space charge oscillation for a given distance can be in- heater is shown in Fig. 2. On the other hand, the chi- signi¯cant at high beam energy. cane provides R52 ¼ ´x ¼ 20 mm,p which will induce a 02 In our study, we take the following two approaches. ppath length di®erence of σ¢z = R52 hx i. Given that 02 In the ¯rst approach, we introduce a density modulation hx i » 20 ¹rad, we have σ¢z » 400 nm À ¸L ´ at the end of injector, where the electron beam energy ¸L=(2¼) ¼ 127 nm. Hence, the energy modulation in- reaches 135 MeV. As what we explained above, at this duced by the laser-heater will be immediately washed energy, and for the modulation wavelength we are in- out by the second half of the chicane, and will not in- terested in, there is no longer a signi¯cant space charge troduce density modulation at ¸L = 800 nm. Therefore, oscillation. In the second approach, we introduce a den- due to this longitudinal position smearing, the induced sity modulation in the electron beam when it was born energy modulation becomes purely slice energy spread; at the cathode. This simulates a realistic situation where and, the laser-heater induced energy modulation will not the electron beam density modulation is due to temporal be converted into any density modulation. modulation of the laser. Details of these two approaches are described below. Approach I: we take a PARMELA output distribution III. SIMULATION DETAILS at the end of injector section at 135 MeV with 200; 000 macroparticles, where there is no density modulation or If the electron beam at birth has density or energy energy modulation in the beam distribution function. We modulations, then due to the longitudinal space charge then superimpose density modulation at speci¯c modu- e®ect, there will be space charge oscillation. Such an lation wavelength and amplitude. In doing so, the longi- oscillation will convert density modulation into energy tudinal momentum correlation is preserved. We also pre- modulation and vice versa. This oscillation essentially serve the correlation of the local slice energy spread. We stops when the electron beam energy reaches certain also preserve the transverse emittance of the beam and value, which is a function of the density modulation wave- its correlations. Under these conditions, we then increase length. For example, assuming an electron beam of rms the total number of particles to 2 million to reduce nu- transverse size of 200 ¹m, and peak current of 100 Amp, merical shot noise. In addition, we use Halton sequence 3 FIG. 1: (Color) Layout of the LCLS accelerator system with Landau damping laser heater at 135 MeV. FIG. 2: (Color) Layout of the LCLS laser heater inside a magnetic chicane at 135 MeV. TABLE III: Summary of the parameters and results for the microbunching and ¯nal slice relative energy spread.